644 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
dington neither claimed nor had anything to do with its introduction ; 
it was Dr. Goring who attached Coddington’s name to it.* Wollaston 
suggested the use of two plano-convex lenses, placed with their plane 
sides in apposition, with a diaphragm between them. Brewster subse- 
quently pointed out that a f great improvement would be effected by 
making each lens a hemisphere ; for then the incident and emergent 
pencils would meet the surfaces normally. Brewster’s next step was to 
simplify its construction by substituting a grooved sphere for the two 
hemispherical lenses. The sphere has been quite given up, and for long 
the construction has been that of a grooved cylinder. 
“ There is yet a further point of interest connected with the achro- 
matism of this form of lens. All are aware that an achromatic effect is 
obtained in an eye-piece by regulating the distance between the two 
converging lenses, made of the same kind of glass, so that the coloured 
images of varying size, formed by the excentrical pencils which fall on 
the first lens, may be unequally magnified by the second lens, in such a 
manner as finally to make uniformly magnified images. These final images, 
being superimposed, and all of one size, form a single image that is 
achromatic for the purposes intended. Now a similar effect can be 
secured in a single lens by separating its refracting surfaces from each 
other ; in other words, by making the lens of a certain thickness. The 
partial achromatism which is obtained in the Coddington lens is therefore 
due to its thickness. 
“ Notwithstanding the theoretical advantages just enumerated, viz. 
first, the normal incidence of the rays, and secondly, the achromatic 
compensation by thickness, it cannot be said that the Coddington is 
altogether a successful form of lens. Its defects, three in number, are, 
smallness of working distance, curvature of image, and want of light. 
There can be no doubt that its performance is incomparably inferior to 
that of the Wollaston doublet. For example, a Wollaston doublet in 
my possession shows tubercle bacilli with considerable clearness. 
“ Returning now, after this long digression, we must admire the 
admirable workmanship of these three little Microscopes. They appear 
to be a part of a doctor’s outfit, and may have been used for the exami- 
nation of blood. The highest power is the smallest Coddington lens I 
have yet seen.” 
Reichert’s New Microscope. — Fig. 150 is an illustration of Reichert’s 
“ Baugh ” Microscope, which was exhibited by Mr. C. Baker at the 
October meeting. It is fitted with the new lever fine adjustment which 
was figured and described on page 374 ante. It should be noted that 
the index, i, for reading the divisions on the head of the micrometer 
screw, is capable of being rotated, so that it can be placed to the zero 
point wherever that may happen to be. 
The foot is lighter, and yet more stable, than those of the usual 
pattern. The stage is of Nelson’s horseshoe form, and the substage is 
of the ordinary Continental side-screw focussing pattern; but it is also 
fitted with centering screws, a a'. The rackwork draw-tube, B, which 
can be screwed into the body, in place of that of the ordinary form, is 
a feature quite novel to modern Continental Microscopes, though it was 
* See Micrographia, p. 182. 
